1. FIELD OF THE INVENTION
The present invention relates to a turbo vacuum pump such that pressure at an outlet port thereof equals to the atmospheric pressure. More particularly, it relates to a compact, easy-to-handle turbo vacuum pump.
2. DESCRIPTION OF THE PRIOR ART
Conventional turbo vacuum pumps are known, such as that disclosed in Japanese Patent Unexamined Publication No. 62-258186. This conventional turbo vacuum pump is equipped with a housing having an inlet port and an outlet port, the housing extending between the inlet port and the outlet port, a rotating shaft rotatably supported with the aid of a bearing in the housing, a centrifugal pump stage and a peripheral pump stage. The pump stages of the above two types are disposed one after another in the housing.
As disclosed in Japanese Patent Unexamined Publication No. 1-267392, it is possible to employ a type of turbo vacuum pump in which pressure in an outlet port thereof is made equal to the atmospheric pressure so as to discharge gas, and in which a magnetic bearing, requiring no lubricating oil, is used as the bearing of the pump.
In the turbo vacuum pump disclosed in Japanese Patent Unexamined Publication No. 1-187396, a centrifugal pump stage and a peripheral pump stage constitute a pumping mechanism portion, and a hydrodynamic type gas bearing supports a rotating shaft.
However, in the turbo vacuum pump disclosed in Japanese Patent Unexamined Publication No. 62-258186 mentioned above, an impeller, a stator plate, another impeller and another stator plate are alternately arranged in the axial direction of the pump. Both of these plates must be divided in half to insert them. Such a structure is complicated, and there is a limit to how small the structure can be made. The pump has a vertical axis structure in which lubricating oil is drawn in from an oil tank at the lower end of the pump so as to lubricate the bearing. Owing to this structure, the number of possible directions from which the pump can be installed is limited. Also, because of the use of an oil-lubricating ball bearing, the oil contaminates the inside of a passage in the pump during long-time use thereof, even though this contamination is negligible.
In the turbo vacuum pump disclosed in Japanese Patent Unexamined Publication No. 1-267392 mentioned above, no contamination caused by oil occurs since lubricating oil is not used. The magnetic bearing, however, requires a large number of very expensive parts, including a control unit. Because the pump has a complicated structure, it is difficult to reduce the size thereof.